Introduces the Statisticser interface which ResultIterators may
implement.
The HTTP implementation uses HTTP trailers to preserve the statistics.
This way we do not need to have all encoders and decoders support
statistics.
It was effectively a copied and pasted platform.ID, so change it to a
type alias. Once our known references to the query/id package are
updated to platform.ID, we'll delete the package.
Nathaniel Cook
Mark Rushakoff